1. Field of the Invention
The present invention relates generally to development of the 3rd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE), and more particularly, to the transmission of sounding reference signals in Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication systems using Time Division Duplexing (TDD).
2. Description of the Art
In order for a communication system to function properly, several types of signals are supported by the system. In addition to data signals, which convey information content, control signals and Reference Signals (RS) also need to be transmitted to enable proper transmission and reception of data signals. Such signals are transmitted from User Equipments (UEs) to their serving Base Station (BS or Node B) in the UpLink (UL) of the communication system and from the serving Node B to UEs in the DownLink (DL) of the communication system. Examples of control signals include positive or negative acknowledgement signals (ACK or NAK, respectively), transmitted by a UE in response to correct or incorrect data packet reception. Control signals also include Channel Quality Indication (CQI) signals providing information about DL channel conditions that the UE experiences. RSs are typically transmitted by each UE to either provide coherent demodulation for data or control signals at the Node B or to be used by the Node B to measure UL channel conditions that the UE experiences. An RS that is used for demodulation of data or control signals is referred to as a Demodulation (DM) RS, while an RS that is used for sounding the UL channel medium, which is typically wideband in nature, is referred to as a Sounding RS or SRS.
A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B (or BS) is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
UEs are assumed to transmit data signals through a Physical Uplink Shared CHannel (PUSCH) while, in the absence of PUSCH transmission, UEs transmit control signals through a Physical Uplink Control CHannel (PUCCH). The data or control signal transmission is over a Transmission Time Interval (TTI) that corresponds to a sub-frame having a duration of 1 millisecond (msec), for example.
FIG. 1 illustrates a block diagram of a sub-frame structure 110 for PUSCH transmission. The sub-frame includes two slots. Each slot 120 includes seven symbols used for the transmission of data signals, RSs, and possibly control signals. Each symbol 130 further includes a Cyclic Prefix (CP) in order to mitigate interference due to channel propagation effects. Signal transmission in different slots may be at the same or a different part of the operating bandwidth. Some symbols in each slot may be used for RS transmission 140 to provide channel estimation and enable coherent demodulation of a received signal. It is also possible for the TTI to have only a single slot or to have more than one sub-frame. The transmission BandWidth (BW) is assumed to include frequency resource units, which are referred to herein as Resource Blocks (RBs). For example, each RB may include NscRB=12 sub-carriers. UEs are allocated one or more consecutive RBs 150 for PUSCH transmission and one RB for PUCCH transmission. The above values are for illustrative purposes only.
In order for a Node B to determine the RBs in which to schedule PUSCH transmission from a UE and the associated Modulation and Coding Scheme (MCS), a CQI estimate of the UL channel medium is required over the PUSCH transmission BW, which is smaller than or equal to the operating BW. Typically, this UL CQI estimate is obtained through the separate transmission of an SRS over the scheduling BW by the UE. This SRS is transmitted in a symbol of an UL sub-frame, replacing the transmission of data or control information. It is used to provide a Signal-to-Interference and Noise Ratio (SINR) estimate over its transmission BW. It can also be used for UL Transmission Power Control (TPC) and UL synchronization.
FIG. 2 shows an SRS transmission. The SRS transmission occurs in a last sub-frame symbol of every other sub-frames 260, 265, for a respective 4.3% SRS overhead. UE1 210 and UE2 220 multiplex their PUSCH transmissions in different parts of the operating BW during a first sub-frame 201, while UE2 220 and UE3 230 do so during a second sub-frame 202, and UE4 240 and UE5 250 do so during a third sub-frame 203. In some symbols of the sub-frame, UEs transmit DM RSs to enable the Node B receiver to perform coherent demodulation of the data or control signal transmitted in the remaining sub-frame symbols. For example, UE1, UE2, UE3, UE4, and UE5 transmit DM RS 215, 225, 235, 245, and 255, respectively. UEs with SRS transmission may or may not have PUSCH transmission in the same sub-frame and, if they co-exist in the same sub-frame, SRS and PUSCH transmission may be located at different parts of the operating BW.
FIG. 3 shows a transmitter structure for the DM RS based on the time-domain transmission of Constant Amplitude Zero Auto-Correlation (CAZAC) sequences. A CAZAC sequence 310 is cyclically shifted in block 320. The Discrete Fourier Transform (DFT) of the resulting sequence is obtained in block 330. The sub-carriers are mapped in block 340 corresponding to the assigned transmission BW of block 350. The Inverse Fast Fourier Transform (IFFT) is performed in block 360. The CP insertion in performed in block 370 and filtering is performed in time windowing block 380, for application to the transmitted signal 390. It is assumed that no padding is inserted by the reference UE in sub-carriers that may be used for signal transmission from other UEs and in guard sub-carriers (not shown). The transmitter structure of FIG. 3 can also be used, possibly with minor modifications (such as the repetition in time of the CAZAC sequence to produce a comb spectrum), for SRS transmission. Moreover, for brevity, additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas, as they are known in the art, are not illustrated.
An alternative generation method for a CAZAC sequence, serving as DM RS or as SRS, is provided in the frequency domain, as illustrated in FIG. 4. With respect to the time-domain generation method of FIG. 3, it is possible that the SRS sub-carriers are not consecutive (SRS has a comb spectrum), which is useful for orthogonally multiplexing (through frequency division) overlapping SRS transmissions with unequal BWs. Such SRS are constructed by CAZAC sequences of different lengths, which cannot be separated using different Cyclic Shifts (CS) as is subsequently discussed. The frequency domain generation of a transmitted CAZAC sequence follows the same steps as the time domain generation with two exceptions. The frequency domain version of the CAZAC sequence is used at block 410. Specifically, the DFT of the CAZAC sequence is pre-computed and not included in the transmission chain. Further, CS block 450 is applied after IFFT block 440. Transmission control bandwidth block 420, sub-carrier mapping block 430, CP insertion block 460, and time windowing block 470 for application to transmitted signal 480, as well as other conventional functionalities (not shown), are the same as FIG. 3.
At the receiver, the inverse (or complementary) transmitter functions are performed. This is illustrated in FIG. 5 and FIG. 6 in which the reverse operations of those in FIG. 3 and FIG. 4 respectively apply.
In FIG. 5, an antenna receives a Radio-Frequency (RF) analog signal and after passing through further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) a digital received signal 510 passes through a time windowing unit 520 and the CP is removed in block 530. Subsequently, the receiver unit applies an FFT in block 540, selects sub-carriers used by the transmitter in block 555 through control of reception bandwidth 550, applies an Inverse DFT (IDFT) in block 560, restores the CS applied to the transmitted CAZAC sequence in block 570 and, using a replica of the CAZAC sequence 580, multiplies (correlates) the resulting signal at multiplier 590 to produce an output 595 which can be used for channel or CQI estimation.
Similarly, in FIG. 6, a digital received signal 610 passes through a time windowing unit 620 and the CP is removed in block 630. Subsequently, the CS of the transmitted CAZAC sequence is restored in block 640, an FFT is applied in block 650, the selection of the transmitted sub-carriers is performed in block 665 through control of reception bandwidth 660, and correlation with a CAZAC sequence replica 680 is subsequently applied at a multiplier 670. Finally, output 690 is obtained and can then be passed to a channel estimation unit, such as a time-frequency interpolator, or an UL CQI estimator.
As described above, the RS (DM RS or SRS) is assumed to be constructed from CAZAC sequences. An example of such sequences is given by the following Equation (1):
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            where L is a length of the CAZAC sequence, n is an index of an element of the sequence n={0, 1, 2 . . . , L−1}, and k is an index of the sequence itself. For CAZAC sequences of prime length L, the number of sequences is L−1. Therefore, an entire family of sequences is defined as k ranges in {1, 2 . . . , L−1}. However, the CAZAC sequences for RS transmission need not be generated by strictly using the above expression. As the RBs are assumed to include an even number of sub-carriers, with 1 RB including NscRB=12 sub-carriers, the sequences used for RS transmission can be generated, in the frequency or time domain, by either truncating a longer prime length (such as length 13) CAZAC sequence or by extending a shorter prime length (such as length 11) CAZAC sequence by repeating its first element(s) at the end (cyclic extension). Alternatively, CAZAC sequences can be generated through a computer search for sequences satisfying the CAZAC properties.
Different CSs of a CAZAC sequence provide orthogonal CAZAC sequences. Therefore, different CSs of a CAZAC sequence can be allocated to different UEs to achieve orthogonal RS multiplexing in the same RBs. This principle is illustrated in FIG. 7. In order for multiple CAZAC sequences 710, 730, 750, and 770, generated respectively from multiple CSs 720, 740, 760, and 780, of the same root CAZAC sequence to be orthogonal, CS value Δ 790 should exceed the channel propagation delay spread D (including a time uncertainty error and filter spillover effects). If TS is the duration of one symbol, the number of CSs is equal to the mathematical floor of the ratio TS/D. For 12 cyclic shifts and for a symbol duration of about 66 microseconds (14 symbols in a 1 millisecond sub-frame), the time separation of consecutive CSs is about 5.5 microseconds. Alternatively, to provide better protection against multipath propagation, only 6 CSs may be used providing a time separation of about 11 microseconds.
The SRS transmission BW may depend on an SINR that the UE experiences in the UL. For UEs with low UL SINR, the serving Node B may assign a small SRS transmission BW in order to provide a relatively large ratio of transmitted SRS power per BW unit, thereby improving the quality of the UL CQI estimate obtained from the SRS. Conversely, for UEs with high UL SINR, the serving Node B may assign a large SRS transmission BW since accurate UL CQI estimation can be achieved from the SRS while obtaining this estimate over a large BW.
Several combinations for the SRS transmission BW may be supported as shown in Table 1, which corresponds to configurations adopted in 3GPP E-UTRA Release 8. The serving Node B may signal a configuration c through a broadcast channel. For example, 3 bits can indicate one of the eight configurations. The serving Node B may then individually assign to each UE, for example using higher layer signaling of 2 bits, one of the possible SRS transmission BWs mSRS,bc (in RBs) by indicating the value of b for configuration c. Therefore, the Node B assigns SRS transmission BWs mSRS,0c, mSRS,1c, mSRS,2c, and mSRS,3c (b=0, b=1, b=2, and b=3, respectively, in Table 1) to UEs having progressively decreasing UL SINRs.
TABLE 1Example of mSRS,bc RB values for UL BW ofNRBUL RBs with 80 < NRBUL ≦ 110.SRS BWconfigurationb = 0b = 1b = 2b = 3c = 09648244c = 19632164c = 28040204c = 37224124c = 46432164c = 56020Not Applicable4c = 64824124c = 74816 84
Variation in the maximum SRS BW is primarily intended to accommodate a varying PUCCH size. The PUCCH is assumed to be transmitted at the two edges of the operating BW and to not be overlapped (interfered) with the SRS. Therefore, the larger the PUCCH size (in RBs), the smaller the maximum SRS transmission BW is.
FIG. 8 further illustrates the concept of multiple SRS transmission BWs for configuration c=3 from Table 1. The PUCCH transmission is located at two edges, 802 and 804, of the operating BW and a UE is configured SRS transmission BWs with either mSRS,03=72 RBs 812, or mSRS,13=24 RBs 814, or mSRS,23=12 RBs 816, or mSRS,33=4 RBs 818. A few RBs, 806 and 808, may not be sounded, but this usually does not affect the ability of the Node B to schedule PUSCH transmissions in those RBs, since the respective UL SINR may be interpolated from the nearest RBs having SRS transmission. For SRS BWs other than the maximum, the serving Node B is also assumed to assign to a UE a starting frequency position of the SRS transmission.
In communication systems using Time Division Duplexing (TDD), DL and UL transmissions occur in different sub-frames. For example, in a frame having 10 sub-frames, some sub-frames may be used for DL transmission and some may be used for UL transmission.
FIG. 9 shows a half-frame structure for a TDD system. Each 5 ms half-frame 910 is divided into 8 slots 920 which are allocated to normal sub-frames, with structure as described in FIG. 1 for UL transmissions, and special sub-frames. A special sub-frame is constructed through 3 special fields: Downlink ParT Symbols (DwPTS) 930, a Guard Period (GP) 940, and Uplink ParT Symbols (UpPTS) 950. The length of DwPTS+GP+UpPTS is one sub-frame (1 msec) 960. The DwPTS 930 may be used for transmission of synchronization signals from the serving Node B, while the UpPTS 950 may be used for transmission of random access signals from UEs attempting to access the network. The GP 940 facilitates the transition between DL and UL transmissions by absorbing transient interference. DwPTS or UpPTS resources not used for the transmission of synchronization signals or random access signals, respectively, may be used for the transmission of data signals, control signals, or RSs.
Assuming that a random access channel consists of Q RBs then, for a UL operating BW of NRBUL RBs and for NRA random access channels, the maximum SRS transmission BW is NRBUL−Q·NRA RBs. For implementation and testing purposes, it is useful that the SRS and the DM RS employ the same CAZAC sequences. Also, because it is useful to avoid large prime DFT lengths, the PUSCH transmission BW and consequently the DM RS sequence length may be constrained to be a multiple of small prime factors such as for example 2α2·3α3·5α5 RBs, where α2, α3, and α5 are non-negative integers. Moreover, if the SRS transmission BW is configured to be a multiple of 4 RBs, as in Table 1, the SRS transmission BW is 2(2+α2)·3α3·5α5 RBs.
Since no PUCCH transmission is assumed in UpPTS symbols, the conventional approach is for a maximum SRS transmission BW NmaxSRS to be NmaxSRS=2(2+α2)·3α3·5α5≦(NRBUL−Q·NRA) RBs. This assumes that NRA random access channels, each comprising of Q RBs, are placed at the two edges of the operating BW, for example, in a similar manner as that for the PUCCH in FIG. 8. For SRS transmission BWs smaller than the maximum, the same values may be maintained regardless of whether the transmission symbol is a UpPTS transmission symbol.
However, the above approach may introduce additional SRS BWs in UpPTS symbols beyond the ones supported in non-UpPTS symbols. For example, for NRBUL=100 and NRA=2, the maximum SRS transmission BW in UpPTS symbols becomes 88 RBs, which is not supported by any configuration in Table 1. Consequently, the number of options for the maximum SRS transmission BW is increased and additional testing is required.
Additionally, the above-described approach does not address situations in which the maximum SRS BW in a UpPTS symbol is smaller than the maximum SRS BW in non-UpPTS symbols.
Additionally, the above-described approach assumes that the random access channels are placed at either one or both of the operating BW edges in a predetermined manner. However, it may be preferable, from an overall system operation standpoint, for a Node B to configure the BW position of random access channels (for example, through broadcast signaling). In such cases, the SRS assignment and the UE behavior regarding SRS transmission should be such that no interference is caused to the transmission of random access signals.